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OGC data service interfaces

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The Open Geospatial Consortium (OGC) provides specifications for components for interoperable geospatial information systems. The main focus is on service interfaces, in support of service-oriented architectures. This topic provides a brief summary of the main informational service interfaces.

In order to use these, it is necessary to understand the scope of the model for the information types that are the primary focus of each interface, so a summary of the main information models is provided at OGCInformationModels.

OGC Web Services - common components (OWS)

The OGC Web Service interfaces use a common concept of operations, specified in the OWS Common Implementation specification [common pattern of requests and responses]. The standard sequence is:
  1. GetCapabilities - to get a basic description of the service, including an indication of the data available, and the operations that are supported
  2. Describe** - to get a detailed description of some available information type, necessary to frame data requests
  3. GetData - a request for data instances, with the selection criteria often expressed using the OGC Filter
The OWS Common specification is available at http://www.opengeospatial.org/standards/common.

The OGC Filter is a syntax for the expression of data selection and reporting rules.

Web Map Service (OGC-WMS)

OGC-WMS provides a standard http request syntax for obtaining a portrayed map.

The main data-request operation is GetMap. It is parameterized by
  1. layers (choose from a list in the GetCapabilities response)
  2. map bounds
  3. layer style (choose from a list provided, per layer, in the GetCapabilities response)
The response is a map, typically in gif, jpeg, png, kml or pdf format.

A OGC-WMS may optionally also support GetFeatureInfo which returns details of a feature identified by layer and location. The response is a textual description of the feature, optionally expressed in a GML Application language.

In a simple OGC-WMS the data offering is bound to the service. However, OGC-WMS may operate as a portrayal module chained in front of another data source, typically a OGC-WFS. The portrayal rules in this case are expressed using a Styled Layer Descriptor document.

A map served by a OGC-WMS can be described in a Web Map Context document. A context document is often used to initialize the state of a OGC-WMS client.

The OGC-WMS specification is available from http://www.opengeospatial.org/standards/wms.

N.B. v1.1 includes SLD support. v1.3 does not include SLD support, but is also published as ISO 19128.

Web Feature Service (OGC-WFS)

OGC-WFS provides a standard http request syntax for fine-grained data access.

The main data-request operation is GetFeature. It is parameterized by
  1. feature-type (choose from a list in the GetCapabilities response)
  2. a projection model, being the feature-properties that should be reported (usually a subset of the properties of the feature-type described in an XML Schema obtained through the DescribeFeatureType operation)
  3. selection criteria (expressed as comparison operations against the feature properties)
The response is a collection of feature instances, in an XML document conforming to a GML Application schema.

Note that spatial selection is expressed as a filter operation with respect to any spatial property of the requested feature-type.

A OGC-WFS may optionally also support Transaction which allows insertion/update/deletion of feature data.

The feature-types served by a particular OGC-WFS may be unique to that OGC-WFS. However, for interoperability purposes it is recommended that a OGC-WFS publish its data using standard feature-type definitions defined in a community application schema. That may require a transformation from a private data model used in the data source.

The OGC-WFS specification is available from http://www.opengeospatial.org/standards/wfs. For a more detailed discussion, see WebFeatureService.

Web Coverage Service (OGC-WCS)

OGC-WCS provides a standard http request syntax for coverage data. In OGC-WCS v1.0 and v1.1 the coverage domain is restricted to grids.

The main data-request operation is GetCoverage. It is parameterized by
  1. coverage identifier
  2. area of interest (expressed as a box)
  3. coverage range property/theme/channel
The response is a coverage, usually in one of the well-known gridded data formats (GeoTIFF, etc) as indicated in the GetCapabilities response.

OGC-WCS v2.0 improves (but does not supersede) previous version 1.1.2. Technical changes from WCS version 1.1.2 include building on the GML 3.2 encoding of coverages, and dividing the standard into this core plus multiple extensions. OGC-WCS v2.0 supports all coverage types supported by said Application Schema; it is not constrained to quadrilateral grid coverages like previous WCS versions.

The WCS specification is available at http://www.opengeospatial.org/standards/wcs

Sensor Observation Service (OGC-SOS)

OGC-SOS provides a standard http request syntax for access to observations and sensor data.

The main data-request operations are GetObservation, DescribeSensor, GetFeatureOfInterest.

A SOS may optionally also support GetResult to obtain just the result of an observation.

The SOS specification is available from http://portal.opengeospatial.org/files/?artifact_id=20994&version=1.

Catalogue Service (OGC-CS/W)

OGC-CS/W provides a standard http request syntax for access to records. It is used primarily for metadata records describing datasets or service instances. For cross-domain discovery purposes a common query profile based on Dublin Core elements is used. However, since the interface is not bound to a specific record model, it can be used for other resources, such as feature-types, SLDs and other resource profiles required to build a spatial data infrastructure (SDI).

The main data-request operations are GetRecords, and DescribeRecord.

The CS/W specifications are available from http://www.opengeospatial.org/standards/cat.

N.B. CS/W is described in the "HTTP protocol binding" clause of the specification.

Interfaces vs. Operations

Each OGC Web Service "interface" is actually just a collection of operations, usually implemented as http requests. Each interface includes those operations necessary to support a coherent interaction sequence, applying the standard OWS pattern of operations to one of the standard information types.

However, for some datasets, it is reasonable to provide alternative views or subsets corresponding to the different standard information types. Access to these would normally be provided by simultaneously deploying multiple different web service interfaces over the data sources.

The following table summarizes the operations available in all the OGC Web Services, indicating which operations are composed into which interface.

Key

Y = Defined in OWS-Common

M = Mandatory

O = Optional

- = Not used in this interface

Operation OWS OGC-CS/W OGC-WMS OGC-WCS v1.x OGC-WCS v2.x OGC-WFS v1.x OGC-WFS v2.x OGC-SOS
GetCapabilities Y M M M M M M M
Get schema for, or description of, resource type
DescribeRecord - M - - - - - -
GetDomain - O - - - - - -
DescribeLayer - - O - - - - -
DescribeCoverage - - - M M - - -
DescribeFeatureType - - - - - M M O
DescribeObservationType - - - - - - - O
DescribeResultModel - - - - - - - O
Get single resource (using ID)
GetResourceById Y - - - - - - -
GetRecordById - M - - - - - -
GetGMLObject - - - - - O - -
DescribeSensor - - - - - - - M
GetObservationById - - - - - - - O
GetResult - - - - - - - O
GetFeatureOfInterest - - - - - - - O
GetFeatureOfInterestTime - - - - - - - O
Get selection from resource(s) (using ID(s) + parameters)
GetMap - - M - - - - -
GetFeatureInfo - - O - - - - -
GetCoverage - - - M M - - -
<nop>Extensions - - - - O - - -
Get collection of resources by search + present (using filter expression)
GetRecords - M - - - - - -
GetFeature - - - - - M M -
GetPropertyvalue - - - - - - O -
GetObservation - - - - - - - M
Stored query support
ListStoredQueries - - - - - - M -
DescribeStoredQueries - - - - - - M -
CreateStoredQuery - - - - - - O -
DropStoredQuery - - - - - - O -
Transaction support
Transaction - O - - - O O -
LockFeature - - - - - O O -
GetFeatureWithLock - - - - - O O -
RegisterSensor - - - - - - - O
InsertObservation - - - - - - - O
Other ops
Harvest - O - - - - - -
GetLegendGraphic - - O - - - - -
GetStyles - - O - - - - -
PutStyles - - O - - - - -

-- SimonCox - 03 Feb 2009

-- DerrickWong - 14 Mar 2011

 
Topic revision: r19 - 14 Mar 2011, DerrickWong
 

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